What causes a person to be blunt and straightfoward?

Sometimes people can be like this when dropping hints, or being subtle or tactful, doesn't work. They get frustrated and feel that they need to be harsh to get their point across.

But sometimes, some people will use the truth, or certain elements of the truth, to deliberately cause pain to others.

I happen to think that people who do this aren't always completely honest in every way, because we need to first be honest with ourselves about the true motives behind the things we say and do. But not everyone can be bothered to take an honest look at themselves. They prefer to point fingers at others instead.